1. Field of the Invention
The present invention is in the field of gear drives, and specifically refers to a gear drive system for producing a rotary motion having a selectable amount of initial delay.
2. The Prior Art
The real articulated steam locomotives were characterized by having two complete drive systems. Each drive system was powered by steam in separate sets of expansion cylinders and capable of operating independently of each other.
It is frequently observed that when a real locomotive starts up, the front set of drive wheels commence to turn before the rear set of drive wheels begin to turn. The purpose of the present invention is to simulate in a model locomotive the action of the drive wheels of a real articulated steam locomotive as it starts up under load from a stop.
In a typical model locomotive, the shaft of an electric motor is directly coupled to a first worm gear that turns a helical gear that is coupled through one or more intermediate gears to a rear drive wheel. A forward extension of the motor shaft turns a second worm gear that drives a helical gear that is directly coupled to the forward drive wheels of the locomotive.
FIGS. 1, 2 and 3 show the drive system used by the present inventor in an earlier effort to produce the desired effect. In that earlier embodiment as shown in FIG. 1, the shaft 12 of an electric motor 14 was directly connected to a 11 worm gear 16 that turned the rear drive wheel 18 through intermediate gears 20 and 22. An extension 24 of the motor shaft was connected through a pair of universal joints 26 and 28 to a second worm gear 30 that was used to turn the forward drive wheel 32.
As best seen in FIGS. 2 and 3, the delayed motion of the rear drive wheel 18 was achieved by providing a pin 34 that extended perpendicular to the face of the first intermediate gear 20 and into a slot 36 in the second intermediate gear 22. The first intermediate gear 20 and the second intermediate gear 22 are mounted on a common shaft 38 but can turn independently of the shaft and are coupled to each other only by the pin 34 as it engages the extremities of the slot 36. The second intermediate gear 22 is always engaged to the drive gear 18, which is directly connected to the rear drive wheel.
This technique of using a pin in a curved slot is shown in FIGS. 3 and 4 of U.S. Pat. No. 4,889,002 of Abraham, who uses the mechanism in a linear actuator.
Although this original approach succeeded in producing the desired delayed motion effect, it was subject to two undesirable limitations. First, if the desired delayed motion were relatively long, the curved slot would have to extend a significant part of the way around the gear, thereby weakening the gear. In addition, such a slot had to be milled into the material of the gear, and this operation required specialized equipment. The second difficulty with the use of the curved slot to produce delayed motion was that the amount of delay was fixed by the length of the curved slot and could not be readily altered.
Pin and slot arrangements were also found in U.S. Pat. No. 2,640,338 of Charvat and in U.S. Pat. No. 2,920,502 of Bungart.
In U.S. Pat. No. 4,274,337, Shaw shows the use of a differential gear mechanism to permit slippage of one set of driving wheels with respect to the other in a model locomotive. The approach used in this patent seems unnecessarily complicated and would permit uncontrollable drive wheel slipping in the drive unit with the least amount of friction against the rails. This action reduces the ability of the model to pull a load. The present inventor set out to find a simpler way of achieving the desired effect.